My main point which I wasn't really clear about is that there are 5 basic scaling modes that would be pretty common, and it's probably best to leave it to the end user as to which they want to use.
The designer will at a baseline design to meet what they see to be the primary use case for themselves or the "client" (friend, compatriot, random web person they imagine using their stuff) first. Beyond that everything else is a nice-to-have. For the cases where the layout isn't thought out for the other cases, it's probably best left to the end user to decide how they want things to scale. Some might want it stretched full screen (damn the aspect ratio). Others fine with letter boxing. I personally loathe stretching, but in setting up a Pi for my buddy he wanted my Ol-Room style 4x3 layouts just stretched for his 16:9, rather than me setting up something else that actually fits correctly.
Some of the scaling also depends on whether the "live area" of content is tied to the positioning of background images or not making things much more complex to think about... in that sometimes parts would be appropriate to stretch and sometimes not.
Anyway, I obviously haven't thought it all through completely. :-) but I have been thinking about it in the context of making my own themes more flexible... and how I plan to approach it.